The present invention concerns a process for synthesizing nonporous high purity silica particles. Such silicas may be further characterized as having skeletal densities of approximately 2.21 g/cc (which are comparable with or nearly comparable with that of vitreous silica), average particle sizes in the range of 3 to 1000 microns, nitrogen B.E.T. surface areas which are less than about 1 m.sup.2 /g, total impurity content of less than about 50 parts per million (ppm) which is inclusive of a metal impurity content of less than about 15 ppm. Since the sizes of these silica particles are measured in microns, it is convenient to call them "micron-sized" silicas.
Non-porous, high purity micron-sized silicas can be used in a variety of applications such as in the production of quartz tubes and crucibles, in the production of optical fibers and in filling epoxy molding compounds. Non-porous, micron-sized silicas are required to attain good flow properties and to attain high packing density in dies in the manufacture of quartz crucibles. Similar properties are required to attain high solids loading levels in epoxy molding compounds. Impurities in the silicas adversely affect product performance properties. The quality of silicon crystals, formed from melts contained in quartz crucibles, can be degraded by impurities in the quartz crucibles. Examples of such impurities include aluminum, boron, alkali metals and transition metals at the parts per million levels. Similarly, contaminants in optical fibers, such as transition metals or silanol groups, cause signal attenuation. Epoxy molding compounds used in encapsulating large capacity dynamic random access memory chips must have low alpha particle emissions. This requires the use of silica fillers having uranium and thorium contents at the low part per billion (ppb) level, since these elements are the source of the alpha emissions.
Many methods are described in the literature for forming dense, high purity silica bodies. These methods, typically, involve sintering to full density or nearly full density, high purity porous silica bodies which are derived from high purity silica precursors. In principle, such processes can be modified to form porous, micron-sized, high purity silica particles which can subsequently be sintered to form the desired products for these uses. Porous particles can be formed by spray drying silica dispersions, by crushing porous silica bodies followed by screening to isolate the desired particle size fraction and by sol-gel processing as, for example, described by Ryon et al. in U.S. Pat. No. 4,459,245 or by Porchia et al. in European Patent Application 0255321. Moreover, because particle mass varies with the cube root of particle size, the average size of the sintered product will, at most, be a factor of 2 smaller than the average size of the porous particles from which they are derived and which have a porosity of less than 85%.
Practical implementation of a process to form the desired non-porous, dense, micron-sized silica particles by sintering porous silica particles requires the simultaneous attainment of at least five criteria. These are:
I. A low cost high purity silica or silica precursor. PA1 II. Avoidance of product contamination during processing. PA1 III. Final product particles containing little or no porosity. PA1 IV. Product particles with little or no silanol content. PA1 V. Rapid processing times. PA1 1) Forming an aqueous dispersion of fumed silica. PA1 2) Optionally, filtering the aqueous dispersion to remove particulate impurities. PA1 3) Converting the aqueous dispersion into porous particles. PA1 4) Optionally, further purifying the silica to remove metal impurities. PA1 5) Sintering the porous particles to nearly theoretical
A sixth optional, but preferred, criterion is that the purity of the silica can be readily upgraded during processing.
The conditions required to attain Criterion II are readily stated. All operations employed in the process to form the desired silica product must be compatible with the use of processing equipment which is constructed of or lined with either a polymeric material or fused quartz. Organic contaminants picked up from the polymeric materials can be burned off prior to or during sintering. Contamination of the products with fused quartz is of little concern. The material of choice in all high temperature operations is fused quartz.
Fused quartz is costly and its use will place a severe constraint on the maximum temperature employed in the process. Fused quartz begins to soften at temperatures above about 1100.degree. C. Moreover, as noted in the Mar. 1970 issue of Design Engineering, the maximum recommended temperature for continuous use of fused quartz is about 1000.degree. C. Only short-term use is recommended at 1300.degree. C. As the temperature is reduced, the use time of fused quartz is extended. In view of the high cost of fused quartz, for practical implementation of a sintering process, the maximum temperature employed should be less than about 1200.degree. C., more preferably, should be less than about 1150.degree. C. and, most preferably, should be less than about 1100.degree. C.
Many groups of workers have described the synthesis of high purity silica glasses using silicas derived from silicon alkoxides such as tetramethyl orthosilicate and tetraethyl orthosilicate. Typically, a silica sol or silica gel is formed by hydrolysis of the alkoxide dissolved in an alcohol such as methanol, ethanol or isopropanol. Since all reagents employed are easily purifiable and since the hydrolysis reaction can be conducted in polymeric vessels, silicas with extremely low levels of inorganic contaminants can be formed.
Sacks and Tseng, in Journal of American Ceramic Society, Vol. 67, 526 (1984) and Vol. 67, 532 (1984), have shown that silicas derived from alkoxides can be utilized to form green bodies which can be sintered to full density or nearly full density at 1000.degree. C. These workers, however, found that residual silanol groups persist at the highest calcination temperature tested, 1050.degree. C.
Matsuyama et al. in Ceramic Bulletin, Vol. 63, 1408 (1984), also studied the synthesis of high purity glasses using silicas derived from alkoxides. These workers found that silanol groups persist in the glass even after sintering at 1300.degree. C. in an atmosphere of helium. The lowest hydroxyl content attained, 360 ppm, was considered to be much too large for optical fiber use. Matsuyama et al. demonstrated that the hydroxyl content can be reduced to negligible levels by subjecting the porous silica to chlorine at temperatures above 800.degree. C. prior to sintering.
Although silicas derived from alkoxides can be extremely pure, can be treated to attain low hydroxyl levels and, probably, can be sintered to nearly full density at 1000.degree. C., their use is not considered in the process of the present invention because they are costly. Their high cost is attributed to the combined cost of the alkoxide and the alcohol used in forming the silicas. Accordingly, such silicas do not satisfy Criterion I set forth above.
Lang et al., In U.S. Pat. No. 4,572,729, describe a process for forming silica articles using high purity SiCl.sub.4 as the silica precursor. In this method high purity water is added to a stoichiometric excess of SiCl.sub.4 to form SiO.sub.2 and HCl gas. After removal of the HCl and the unreacted SiCl.sub.4 (by heating to about 900.degree. C.), the resulting silica was described as having a particularly low hydroxyl content. The silica was molded into a desired shape and then sintered at temperatures in the range of 1000.degree. C. to 1300.degree. C. The resulting body, which presumably was porous, was superficially fused by the action of a hydrogen-oxygen flame which burns at a temperature exceeding 1300.degree. C. Accordingly, in view of the high sintering temperature required to sinter to full density, the process of Lang et al., because of Criterion II, cannot be readily modified to make non-porous, large particle size, high purity silica.
Other workers have described the synthesis of high purity silica glasses using fumed or pyrogenic silicas produced by the flame hydrolysis of silicon tetrachloride, chlorosilanes, organic silicon compounds and mixtures thereof. As pointed out by Clasen, in Journal of Materials Science Letters, 7, 477 (1988), fumed silicas represent inexpensive starting materials which are produced on an industrial scale. Scherer, in Journal of the American Ceramic Society, 60, 236 (1977), indicates that silicas produced by flame hydrolysis can contain less than 10 ppm total impurities. Further, Clasen showed that treatment of a fumed silica having less than 1 ppm each of Na, K, Fe, Ni, Cr, Cu, Co, Mo and Zr in an atmosphere containing SOCl.sub.2 and O.sub.2 at 1100.degree. C. reduces the impurity levels of Fe, Co, Ni, Cr and, probably, Cu and Mo to the ppb (parts per billion) level. Clasen also implies that the treatment reduces the hydroxyl content of the silica.
The formation of dense silica bodies from fumed silica, typically, requires the use of sintering temperatures in excess of 1200.degree. C. For example, Rabinovich et al., in Journal of the American Ceramic Society, 66, 683 (1983), 66, 688 (1983) and 66, 693 (1983), produced green bodies from aqueous fumed silica dispersions containing 40 weight % solids. These bodies could be sintered to nearly theoretical densities only at temperatures in excess of 1300.degree. C. The sintering atmosphere employed had a bearing on the hydroxyl concentration in the sintered body. For example, a helium atmosphere containing 3%, by volume, chlorine was found to be useful in removing bound hydroxyl groups.
Clasen formed other fumed silica dispersions containing up to 55 weight % solids. Green bodies derived from these dispersions were zone-sintered at 1500.degree. C.
Rabinovich, in Journal of Materials Science, 20, 4259 (1985), reviewed the preparation of glass articles by sintering. He indicated that defect-free glasses can be prepared from fumed silicas, using an undisclosed process, by sintering at temperatures as low as 1260.degree. C. to 1300.degree. C. Dehydration in a chlorine containing atmosphere reduced hydroxyl concentration to below 1 ppm.
The above discussion demonstrates that although fumed silicas can serve as an excellent source of high purity silica, green bodies derived from such silicas do not sinter to nearly theoretical densities, at least under conventional conditions, at temperatures of less than 1200.degree. C.
It is known that humid atmospheres markedly accelerate the sintering rates of porous silica bodies. Scherer, in Journal of American Ceramic Society, 60, 239 (1977), and Tseng et al., in Journal of Materials Science, 21, 3615 (1986), attribute the enhanced sintering rate to the water vapor interacting with the silica to form silanol groups.